According to JP-B2-3562938, an electronic throttle control apparatus is provided to a vehicle for controlling an internal combustion engine. In this throttle control apparatus, a throttle valve collides against a full close stopper immediately after turning an ignition switch ON, so that the throttle control apparatus learns a full close position as a reference position of the throttle valve in accordance with a detection signal of a throttle position sensor. Driving duty applied to the throttle motor, such as a DC motor is controlled using a PID algorithm, for example, such that actual opening degree of the throttle valve coincides with target opening degree of the throttle valve on the basis of the full close position, as the reference position, in an engine operating condition. The target opening degree of the throttle valve is set on the basis of an accelerator position, for example. The actual opening degree of the throttle valve is detected using the throttle position sensor.
In this structure, an error may arise in a learning operation of the full close position, which corresponds to the position of the full close stopper, due to an error arising in detection of the throttle opening sensor. Accordingly, when the target opening degree is set at the full close position, the learning value of the full close position may be displaced to the closing side with respect to the actual point of the full close position. When the learning value is displaced to the closing side, the driving operation of the throttle valve to the closing side may be continued by the PID control after the throttle valve collides against the full close stopper in the full close position. In this condition, deviation between the actual opening degree of the throttle valve and the target opening degree of the throttle valve may not decrease. As a result, the driving duty for operating the throttle motor may immediately increase to the maximum driving duty such as −100%, and excessive current may flow through a winding of the throttle motor. When this condition continues, the winding of the throttle motor may cause a failure such as burnout.
In view of the above problem, it may be determined that the throttle motor is in a fail condition when the maximum driving duty applied to the throttle motor is maintained for a predetermined period. In this fail condition, the operation mode may be switched to a failsafe mode, in which electricity supplied to the throttle motor may be terminated, and the throttle valve may be mechanically operated in conjunction with the operation of the accelerator pedal. In JP-B2-3562938, the learning value of the full close position is biased during the engine operation, so that the throttle valve may be controlled such that the throttle valve does to collide against the full close stopper, which corresponds to the actual point of the full close position. In this operation, driving the throttle valve in the vicinity of the full close position can be prohibited. Alternatively, the learning value of the full close position may be corrected every time in a condition, in which the throttle valve makes contact with the full close stopper, during the engine operation.
However, in recent years, enhancement in fuel efficiency of the engine and reduction in emission are progressively required. Accordingly, it is required to enhance a control performance of the throttle valve in the vicinity of the full close position for reducing idling engine speed when the engine rotation speed is extremely low. However, it is difficult to satisfy these requirement using the above structures and operations. Specifically, in JP-B2-3562938, the throttle valve is controlled such that the throttle valve does to collide against the full close stopper when the learning value of the full close position is displaced. Accordingly, it is difficult to control the throttle valve in the vicinity of the full close position.
According to the above operation, the operation mode may be switched to the failsafe mode when the maximum driving duty continues for a predetermined period. However, when the throttle valve is controlled in the vicinity of the full close condition for a long period, errors may arise in the learning operation of the full close position and in detection of the throttle opening sensor. Accordingly, the throttle valve may collide against the full close stopper due to these errors, and the throttle control apparatus may be determined to be in the fail condition. Consequently, in this operation, the throttle valve needs to be restricted from controlling in the vicinity of the full close position.
Furthermore, according to the above operation, the learning value of the full close position may be corrected every time in a condition, in which the throttle valve makes contact with the full close stopper, during the engine operation. However, even in this operation, the learning operation of the full close position may cause a slight error. In addition, an error may arise in detection of the throttle opening sensor. Accordingly, in this operation, the correcting operation of the full close position may be frequently repeated when the throttle valve is controlled in the vicinity of the full close condition for a long period. Accordingly, the full close condition may become unstable, consequently the control of the throttle valve also may become unstable.